There Is Always Risk In Portability

Wednesday, May 7, 2008

roll the dice with LINQAfter my last post, someone asked me if the "portable" repository pattern was really a good idea. He was referring to the fact the LINQ queries in the MVC Storefront and Background Motion applications would sometimes execute against in-memory collections (for unit testing), while the rest of the time the queries would execute against a relational database. Isn't there a huge risk in developers not knowing if the software really works with the database?

I don't think of the repository as a "portability" layer, although since it is an abstraction layered on top of the data access code it can provide some nice indirections, like the ability to switch the persistence store. Is this risky? Sure, there is always some element of risk in portability. Just ask anyone who has written code with a portable UI toolkit, or in HTML for that matter. You don't know what is going to happen until the 1s and 0s hit the silicon.

But …

That's not the job for unit tests. Ideally, you'll have some other tests to verify what happens when the "production" code runs.

Before continuing, I must say that in the last post I neglected to tell you that the brainy Mindscape team and Andrew Peters are responsible for the Background Motion web site, and the code that powers the site. Make sure to visit the site and marvel at the beauty of New Zealand, then drop into the Mindscape blogs. Everyone - let's hear it for New Zealand!

What Is This Risk You Speak Of?

You can write a LINQ query that works fine against in-memory collections, but that can fail spectacularly when you swap in a remote LINQ provider. Here is an obvious example:

var result =
    from a in dc.Addresses
    where !String.IsNullOrEmpty(a.PostalCode)
    select a;

This query is happy to execute using LINQ to Objects, but it fails with an exception if LINQ to SQL is sitting behind the sequence (NotSupportedException: Method 'Boolean IsNullOrEmpty(System.String)' has no supported translation to SQL).

Those types of problems are easy to spot in automated integration testing because exceptions are relatively easy to track down. The real risk is in the queries that don't flame out in spectacular fashion, but execute successfully with slight variations. Here is one example:

var distinctPostalCodes =
       (
        
from a in addresses
        
orderby a.State ascending
         select
new { a.PostalCode, a.State }
       ).Distinct();

This query wants to get distinct list of zip codes and states for all our customers, and order the list by state. Works perfectly with LINQ to objects, and executes successfully in LINQ to SQL. Just one tiny problem you might observe in the generated SQL:

SELECT DISTINCT [t0].[PostalCode], [t0].[State] FROM [dbo].[Address] AS [t0]

Notice the distinct (pardon the pun) lack of an ORDER BY clause. If the upper layers were expecting the results sorted by State then we have problems.

It turns out that LINQ to SQL throws out an inner OrderBy operator when the Distinct operator comes into play. This could be for several reasons, but the most likely reason is DISTINCT and ORDER BY have an uneasy relationship in ANSI SQL (it's not just MS SQL). You can read more about this on Jeff Smith's blog: SELECT DISTINCT and ORDER BY, and there is another good explanation here: Some Common Mis-conceptions about DISTINCT.

One also has to wonder if Distinct might reorder the results in its quest to remove duplicates - it's not explicitly documented that it doesn't. In this case, it's better to forego the query comprehension syntax and make the pipeline of operators more explicit:

var distinctPostalCodes =
      addresses.Select(a =>
new { a.PostalCode, a.State })
               .Distinct()
               .OrderBy(a => a.State);

This forces LINQ to SQL to generate a safe query with the expected results.

Is there risk? Sure – and it's not just in LINQ to SQL. Any multi-target technology runs the same risk. You just need an awareness and safety net (in the form of tests) to mitigate the risk.


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